The present invention relates to an MRI (Magnetic Resonance Imaging) apparatus, and more particularly to an MRI apparatus which is capable of producing a diffusion-weighted image of a subject in a short scan time without being affected by a slight motion of the subject.
FIG. 5 is an explanatory diagram of the diffusion-weighted EPI (Echo Planar Imaging) pulse sequence, with two-dimensional navigation echoes being added thereto, which is disclosed in xe2x80x9cDiffusion-Weighted Interleaved Echo-Planar Imaging with a Pair of Orthogonal Navigator Echoesxe2x80x9d, Kim Butts et al, MRM 35:763-770 (1996).
In this diffusion-weighted EPI pulse sequence Axe2x80x2, an excitation RF pulse R90 and a slice-select pulse S1 are applied, an MPG (Motion Probing Gradient) pulse MPG1 is applied next, an inverted RF pulse R180 and a slice-select pulse S2 are applied next, and an MPG pulse MPG2 is applied next. Subsequently, in the presence of application of a phase-encoded pulse PE to the phase-encoding axis and the absence of application of a read pulse to the read axis, data for the correction of y-direction is collected from the navigation echo y. Subsequently, in the presence of application of a phase-encoded pulse PE to the phase-encoding axis and application of a read pulse RDxe2x80x2 to the read axis, data for the correction of x-direction is collected from the navigation echo x. Subsequently, in the presence of application of a phase-encoded pulse PE to the phase-encoding axis and the inverted version of the read pulse RDxe2x80x2, data for imaging is collected from echoes exe2x80x2. This series of pulse application and data collection is repeated.
The above-mentioned diffusion-weighted EPI pulse sequence Axe2x80x2 is repeated a number of times thereby to collect data necessary for image reconstruction based on the number of times of excitation.
Subsequently, the phase shift among the imaging data which have been collected by the number of times of the diffusion-weighted EPI pulse sequence Axe2x80x2 is corrected by use of the navigation echoes y and x.
A diffusion-weighted image is produced from the corrected data based on the two-dimensional Fourier scheme.
There is disclosed a diffusion-weighted imaging method using the radial scanning method in xe2x80x9cAnalysis and Comparison of Motion-Correction Technique in Diffusion-Weighted Imagingxe2x80x9d, Theodore P. Trouard et al., JMRI 6:925-939 (1996).
This diffusion-weighted imaging method based on the radial scanning method repeats a pulse sequence, in which a gradient pulse is applied so that the gradient magnetic field has a direction of xcex8i and data is collected, while varying the xcex8i, thereby collecting data necessary for image reconstruction based on the number of times of excitation.
A diffusion-weighted image is produced from the data based on the projection reconstruction scheme.
The foregoing prior art diffusion-weighted EPI pulse sequence Axe2x80x2, with two-dimensional navigation echoes being added thereto, uses the navigation echoes y and x to correct the phase shift caused by a slight motion of the subject in the y and x directions. However, it has no ability to cope with a phase shift caused by a slight motion of the subject in the z direction, and there arises actually a problem of the influence of a slight motion of the subject appearing in the image.
In contrast, the foregoing prior art diffusion-weighted imaging method using the radial scanning method is advantageous in that it is not affected by the phase shift caused by a slight motion of the subject owing to the use of only frequency encoding. However, it involves a problem of a long scan time for the collection of data necessary for image reconstruction based on a number of times (64 or more) of excitation.
Accordingly, it is an object of the present invention to provide an MRI apparatus which is capable of producing a diffusion-weighted image of a subject in a short scan time without being affected by a slight motion of the subject.
At a first viewpoint, the present invention provides an MRI apparatus which comprises an RF pulse transmission means, a gradient pulse application means, an NMR signal reception means, and a processing means which operates on these means to collect data and reconstruct an image, the processing means basing the operation on a fast pulse sequence which is a fast pulse sequence of the projection restoration scheme, with MPG pulses being incorporated therein, thereby collecting data necessary for image reconstruction based on the single excitation or the repetition of the fast pulse sequence a number of times thereby to collect data necessary for image reconstruction based on the number of times of excitation.
The term xe2x80x9cfast pulse sequencexe2x80x9d mentioned above signifies a pulse sequence by which data is collected from multiple echoes based on the single excitation.
The MRI apparatus of the first viewpoint, which is based on the projection restoration scheme using only frequency encoding, is not affected by the phase shift caused by a slight motion of the subject. It uses a fast pulse sequence for collecting data from multiple echoes based on the single excitation, and therefore the scan time can be reduced. Based on the fast pulse sequence which incorporates MPG pulses, a diffusion-weighted image can be obtained. In consequence, it becomes possible to obtain a diffusion-weighted image in a short scan time without being affected by a slight motion of the subject.
In the case of collecting data necessary for image reconstruction based on the single excitation, the scan time can be minimized.
On the other hand, in the case of collecting data necessary for image reconstruction based on a number of times of excitation, only echoes having large signal levels are used (it is unnecessary to use echoes of small signal levels) , and therefore the s/n characteristics can be improved. The number in the term xe2x80x9ca number of timesxe2x80x9d mentioned here is equal to a number of pieces of data necessary for image reconstruction divided by a number of pieces of data obtained by one excitation (i.e., number of echoes used).
At a second viewpoint, the present invention provides an MRI apparatus which comprises an RF pulse transmission means, a gradient pulse application means, an NMR signal reception means, and a processing means which operates on these means to collect data and reconstruct an image, the processing means basing the operation on a fast pulse sequence, in which an excitation RF pulse and a slice-select pulse are applied, an MPG pulse is applied next, an inverted RF pulse and a slice-select pulse are applied next, an MPG pulse is applied next, and a read pulse is applied next to collect data so that the read gradient has a direction of angle xcex8i of a virtual radial line on the slice plane, with this series of pulse application and data collection being repeated while reversing the polarity of the read pulse and varying the angle xcex8i, thereby collecting data necessary for image reconstruction based on the single excitation or the repetition of the fast pulse sequence a number of times thereby to collect data necessary for image reconstruction based on the number of times of excitation.
The MRI apparatus of the second viewpoint uses a fast pulse sequence for collecting data by application of a read pulse so that the read pulse has a direction of angle xcex8i, and repeats the pulse sequence while varying the angle xcex8i. This fast pulse sequence is a fast pulse sequence based on the projection restoration scheme which uses only frequency encoding, and therefore it is not affected by the phase shift caused by a slight motion of the subject and it can reduce the scan time. Based on the fast pulse sequence which incorporates MPG pulses before and after the inverted RF pulse, a diffusion-weighted image can be obtained. In consequence, it becomes possible to obtain a diffusion-weighted image in a short scan time without being affected by a slight motion of the subject.
At other viewpoint, the present invention provides a method of diffusion-weighted imaging based on a fast pulse sequence, with MPG pulses being incorporated therein, the method collecting data necessary for image reconstruction based on the single excitation or the repetition of the fast pulse sequence a number of times thereby to collect data necessary for image reconstruction based on the number of times of excitation.
The present invention also provides a method of diffusion-weighted imaging, in which an excitation RF pulse and a slice-select pulse are applied, an MPG pulse is applied next, an inverted RF pulse and a slice-select pulse are applied next, an MPG pulse is applied next, and a read pulse is applied next to collect data so that the read gradient has a direction of angle xcex8i of a virtual radial line on the slice plane, with this series of pulse application and data collection being repeated while reversing the polarity of the read pulse and varying the angle xcex8i, thereby collecting data necessary for image reconstruction based on the single excitation or the repetition of the fast pulse sequence a number of times thereby to collect data necessary for image reconstruction based on the number of times of excitation.
The MRI apparatus of this invention, which is based on the projection restoration scheme using only frequency encoding is not affected by the phase shift caused by a slight motion of the subject. It uses a fast pulse sequence and accordingly can reduce the scan time. Based on the fast pulse sequence which incorporates MPG pulses, a diffusion-weighted image can be obtained. In consequence, it becomes possible to obtain a diffusion-weighted image in a short scan time without being affected by a slight motion of the subject.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.